The invention relates to an electric filter circuit wherein a self enclosed line loop (or closed line loop) having the phase constant 2n.pi. (n=1,2 . . . ) at the required pass middle frequency, is connected at various points, in each case to at least one supply line and at least one line for the withdrawal of the electric signals, and wherein the closed line loop determines the frequency-dependent transmission characteristics of the filter circuit which is constructed in the integrated semiconductor technique as a bucket brigade device (BBD) circuit controlled by a pulse generator, or as a charge coupled device (CCD) circuit, and the image impedance of the self-enclosed line loop is selected to differ from that of the supply- and withdrawal-lines, and furthermore for the self enclosed line loop there are provided input couplings and output couplings which, together with a part of this loop and with integrated amplifiers having predetermined amplification factors, are designed in the topology of a lattice- or a .pi.-element, in such manner that the line loop, together with the input coupling and output coupling, forms a four-arm branching circuit with the equivalent circuit diagram of a reactance branching circuit and when such branching circuits are connected in a chain circuit or cascade circuit, connection lines are provided which, together with a part of the output couplings and input couplings by which they are adjoined, form a further self-enclosed line loop which represents an intermediate coupling loop and which possesses the phase constant (2n-l).pi. at the required pass middle frequency.
One of the disclosures of application U.S. Ser. No. 630,932 filed Nov. 11, 1975 and of the corresponding German Pat. No. 24 53 669 consists of electric filter circuits which can be constructed using self enclosed line loops exhibiting unidirectional transmission characteristics. The lines used can also be so-called CTD (charge transfer device) lines. Lines of this type are known per se as so-called bucket brigade device circuits, or also as so-called CCDs (charge coupled devices). The operation of these circuit components requires pulse generators as likewise described in detail in the German Pat. No. 24 53 669.
German application Ser. No. P 25 34 319.5 illustrates a possible realization of a CTD (charge transfer device) circuit of the foregoing type. This circuit can be used as input coupling and output coupling circuit for the self enclosed line loops of CTD filter circuits of this kind. Another possible realization of input coupling and output coupling circuits which is designed in the form of a .pi.-element has been proposed in German application Ser. No. P 26 08 582.5. Still another possible realization of the input coupling and output coupling circuits is described in detail in the earlier German application Ser. No. P 26 08 540.5. A characteristic of these circuits is that, as it were, an image impedance jump is simulated, the effect of which is known per se from the microwave technique. For this reason, the individual connection lines possess different recharging capacitances.
In order to construct a multi-circuit filter circuit arrangement, it is now necessary to connect in a chain or cascade arrangement a plurality of resonance structures formed as four arm branching circuits. Circuit of this type can be represented in their electric equivalent circuit diagram as loss-free reactance branching circuits and dimensioned accordingly, as stated for example in the magazine "NTZ" 1963, edition 6, pages 297 to 302. The circuit details of a cascade connected multi-circuit filter arrangement are described in the aforementioned German patent applications. In order to connect successive line loops, it is necessary to provide a further self enclosed line loop (self closed line loop) which may be termed an intermediate coupling loop.
Investigations on which the invention has been based show that in the above described circuits, close to the resonance frequency of the self enclosed intermediate coupling loop, oscillations occur when, for example, the amplifiers thereof possess, due to production tolerances or temperature dependences, higher amplification factors than indicated by calculations. Although these oscillations do not lie within the desired pass band they act as an self-oscillation because they are oscillations which occur even when no input signal is present.